Frequency-mixing of optical waves has traditionally been achieved using crystals with non-inversion symmetric lattices. In the case of three-wave mixing, two waves of frequency .omega..sub.1, .omega..sub.2 are mixed to produce another optical wave at either the sum or the difference frequency .omega..sub.3 =.omega..sub.1 .+-..omega..sub.2. The efficiency of the process is governed by two conditions:
(i) The crystal must have a high second-order non-linearity, which initiates the frequency-mixing process; and PA0 (ii) The effective refractive indices (or propagation constants) of the three waves must be matched in order to ensure that the waves propagate in-phase inside the crystal, a condition which is commonly referred to as phase-matching.
It has recently been shown that efficient frequency-mixing, in particular second-harmonic generation (whereby a pump-wave at frequency .omega. is converted into a second-harmonic at frequency 2.omega.), may be obtained in spatially-prepared optical fibre waveguides (Osterberg, U. and Margulis, W.: "Dye-laser pumped by Nd: Yag laser pulses frequency-doubled in a glass optical fibre", Opt. Lett., 1986, 11, p 516; Farries, M. C. et al: "Second-harmonic generation in an optical fibre by self-written-grating", Electron. Lett. 1987, 23, p 322; Stolen, R. H. and Tom, H. W. K: "Self-Organisation phase-matched harmonic generation in optical fibres", Opt. Lett., 1987, 12, p 585). The fibres are usually prepared by exciting them simultaneously with intense radiation at two different wavelengths, a fundamental and the second harmonic, e.g. 532 nm and 1.064 .mu.m. This process has been shown to produce a permanent spatially-periodic second-order susceptibility (.chi..sup.(2)) in the fibre. The efficiency of of this process has been up to 10% for an input peak power of 1 kw (Farries M. C. "Efficient second-harmonic generation in an optical fibre", Proc. Colloquium on Non-Linear Optical Waveguides, London IEE 1988).
It is believed that the second-order susceptibility arises from the orientation of multi-photon-excited defect centres under the influence of a self-induced internal dc-field. The internal field is generated by a third-order nonlinear process involving both the exciting radiations at 1.064 .mu.m and at 532 nm.
Recently, it has been shown that a much greater second-order susceptibility may be produced in a fibre by applying a large (&gt;100 V/.mu.m) external dc electric-field across the fibre at the same time as defects are being excited by intense blue light propagating in the core in a guided mode (Bergot, M. V. et al: "Generation of permanent optically-induced second-order non-linearities in optical fibres by poling"), Opt. Lett., 1988, 13, p. 592). The increase in x(.sup.2) is due to the much larger electric field inside the fibre. However, in this reported experiment, second-harmonic conversion efficiency was very low, since no phase-matching between applied infra-red wavelength waves was achieved.
A degree of phase-matching has since been demonstrated using a non-periodic second-order non-linearity (Fermann, M. E. et al: "Frequency-doubling by modal phase-matching in poled optical fibres", Electron. Lett. 24, 1988, p. 894). Here phase-matching has been achieved by exploiting the phase velocity difference which occurs between the pump in the lowest-order guided mode and the second-harmonic in a higher-order guided mode. This technique has the disadvantage that is is extremely sensitive to the small changes in fibre parameters along the length.